This invention relates to a heat transfer sheet having high preservability and high thermal sensitivity.
An ink jet method and a heat transfer printing method are now rapidly replacing conventional recording and printing methods. This is because a monochromic or full-colored image having high quality can be simply and promptly produced by these methods. Of these methods, a sublimation-type heat transfer printing method utilizing a sublimable dye is far superior to the other methods because a full-colored image with an excellent continuous gradation, comparable to a photographically obtainable image, can be obtained by this method.
A heat transfer sheet prepared by providing a dye layer (heat transfer layer) containing a sublimable dye and a binder on one surface of a substrate film such as a polyester film, and a heat resistive layer on the other surface of the substrate film, which prevents the heat transfer sheet from adhering to a thermal head upon heat transfer printing, is generally used with the sublimation-type heat transfer printing method. Together with the heat transfer sheet is used a heat transfer image-receiving sheet which has a dye-receiving layer comprising a polyester resin. To conduct heat transfer printing, the heat transfer sheet is superposed on the heat transfer image-receiving sheet so that the dye layer faces the dye-receiving layer, and thermal energy is then applied by a thermal head to the back surface of the substrate film of the heat transfer sheet. The sublimable dye contained in the dye layer of the heat transfer sheet thus transfers imagewise to the dye-receiving layer of the heat transfer image-receiving sheet to produce therein a desired image.
When the sublimation-type heat transfer printing method is carried out, only the sublimable dye contained in the dye layer of the heat transfer sheet transfers to the heat transfer image-receiving sheet, and the binder in the dye layer remains on the substrate film. A sharp image with high density can be obtained when the dye has high transferability.
A conventional heat transfer sheet exhibits high transferability of dye right after the production thereof. The transferability of dye, however, gradually deteriorates as time goes by. As a result, the heat transfer sheet which has been preserved for a long period of time produces an unfavorable image with low density. A conceivable reason for the above is that the dye, which exists in the binder in a homogeneously dissolved or finely dispersed state when the heat transfer sheet is prepared, is gradually separated from the binder and crystallizes with the passage of time.
A binder which can readily dissolve therein a sublimable dye and has high hydrophobicity can eliminate the above shortcoming. Namely, when such a binder is employed, the dye will not be easily separated from the binder. In this case, however, high transferability of dye cannot be expected because the binder has an excessively high affinity for the dye.
On the other hand, when a binder having high water solubility is used, the dye, which is generally insoluble in water, cannot be dissolved but is only dispersed in the binder. As a result, the dye cannot easily transfer to a heat transfer image-receiving sheet even when heat transfer printing is carried out by using a fresh heat transfer sheet. In addition, the heat transfer sheet comprising such a binder in its dye layer cannot endure long-term preservation.